Logic circuits for interfacing to a bus within a larger system, such as a CPU or a PC, are well known. As system performance has increased, the trend has been toward using combined complementary MOS (CMOS) and bipolar transistors, commonly known as BiCMOS. The major advantages are high speed and reduced power dissipation. In the typical BiCMOS circuit, CMOS devices form the input and the bipolar devices form the output. A commercially available family of such devices from Philips Components-Signetics is known as the ABT Advanced BiCMOS Interface Logic. All of the devices in this family will operate generally with a 5 volt DC supply voltage source. Usually, the circuit output is connected to, directly or indirectly, for driving, a bus common to a number of other circuits, and the input is derived from a part of the CPU or a peripheral device. The significant point is that, during operation, the bus voltage swings between the voltage source, typically 5 volts, and a reference voltage, such as ground, and these transitions are interpreted as signals by interface circuits connected to the bus.
A traditional zero-static power BiCMOS driver creates a high-to-low transition by channeling current from the driver output into the base of a pull-down bipolar transistor. This configuration is automatically zero-static power because the pull-down transistor starts turning off as the output is pulled towards a bipolar base-emitter drop VBB above ground, but the bipolar device cannot pull the output all the way to ground. Typically, an NMOS device is added to pull the output the rest of the way down. Unless a very large NMOS device consuming valuable semiconductor real estate is used, however, the last volt or so of the high-to-low transition takes a relatively long time, which depends on the load. This is undesirable for many applications.
Two attributes of desirable bus drivers are Incident Wave Switching and Glitch Suppression. Incident Wave Switching is a property of transmission lines. When the logic level on one side of a transmission line is switched, an electrical wave equal to some fraction of the total charge is launched onto the line. After some time, the line settles into the new state. Incident Wave Switching means that a valid logic level is attained with the first, or incident, electrical wave launched onto the line, thereby saving time for the bus user.
Glitch Suppression is the ability of the bus driver to maintain the integrity of a logic level on the bus. This is desirable because noise on the bus may disrupt the operation of the driver.
Known driver circuits allow Incident Wave Switching by sinking large bus currents quickly to ground.
A problem, however, is that, with the known circuit, under certain load conditions, the circuit reacts too quickly to sink large currents from the output, with the undesired result that ringing, or noise, is created, which can interfere with the proper operation of the driver circuit. Furthermore, these circuits generally do not have high current-sinking capability at low output voltages.